The present invention relates to a method for drilling at least one hole into a workpiece.
Drilling into a workpiece is difficult, among other things, when the same has one or more cavities or, generally speaking, wall sections, which are arranged offset behind one another. The rear wall section, as seen looking in the drilling direction, for example, impairs drilling in the front wall section. In addition, measures must be taken which prevent damage to this wall section when the penetration is made in the front wall section. Workpieces that are this difficult to drill exist in the form of turbine blades, for example, in which a plurality of holes are to be provided for cooling.
It is known to drill holes into such workpieces by way of laser or electrical discharge machining (see, for example, U.S. Pat. No. 7,041,933 B1). These methods have the disadvantage that the material ablation takes place by heat development, which may result in undesirable damage to sensitive layers. Electrical discharge machining has the further disadvantage that it can only be used for conductive workpieces.
A known alternative is that of using liquid machining jets for drilling. This type of machining has the advantage that no heat develops during drilling and non-conductive workpieces can also be machined. It is known from EP 1 408 196 A2 to introduce the machining head, from which the machining jet exits during drilling, into a cavity of the workpiece and to drill the hole from the inside out. This method has the disadvantage that it can only be used for special geometries of workpieces and holes. Drilling is in particular not possible when the cavity is not accessible to the machining head and/or the drilling direction is oriented perpendicularly to the workpiece surface, for example.
From U.S. Pat. No. 4,955,164 a method for drilling a hole by means of an abrasive jet acting permanently on the workpiece is known. Thus, it is difficult to stop the impact of the jet precisely when it penetrates the workpiece.
A method is disclosed in WO 92/13679 A1, wherein an ultrasonic generator is used to produce cavitation bubbles in a machining jet formed from pure water. The disclosed method is not suitable to drill holes in a workpiece such that undesirable damages are prevented.